Machining fixture production

ABSTRACT

Disclosed is a method of producing a machining fixture for fixedly holding a workpiece during machining of that workpiece. The method comprises: providing an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined; determining a datum, the datum being dependent upon the relative positions of the receiving elements; determining positions and orientations of one or more reference surfaces with respect to the datum; measuring the surface of the initial machining fixture with respect to the datum; and, thereafter, controlling machining apparatus with respect to the datum to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture.

FIELD OF THE INVENTION

The present invention relates to machining fixtures and the production thereof.

BACKGROUND

Aircraft airframes typically comprise a plurality of frames (or formers) and longerons (or stringers/stiffeners). The frames are typically laterally spaced from one another and arranged perpendicular to the longitudinal axis of the aircraft. The primary purpose of formers is to establish the shape of the fuselage and reduce the column length of the longerons. The longerons are typically elongate members which are attached to the frames and are arranged parallel to the longitudinal axis of the aircraft. The longerons support the aircraft skin and, in use, transfer aerodynamic loads acting on the skin onto the frames.

It is desirable that aircraft airframes are produced to be within very tight tolerance bounds.

Production of an aircraft airframe typically comprises producing two or more separate airframe sections (for example, a fore fuselage section, an aft fuselage section, and a tail section), and subsequently attaching those sections together.

It tends to be very difficult to produce separate airframe sections with a sufficient level of precision to allow for easy assembly of the airframe. Lengthy and expensive shimming processes may be required to fill gaps between the airframe sections when those sections are attached together.

Production of a section of an aircraft airframe typically involves the use of airframe assembly tools designed to support airframe components while they are being worked on and to locate different components together in the correct relative positions during airframe assembly. Traditionally, each different assembly process has required at least one dedicated assembly tool, which is produced specifically for a given set of components and which is designed to support the components in a particular manner so that assembly operations can be carried out without interference from the tool. Such assembly tools are manufactured to exacting standards.

A conventional assembly tool comprises a rigid metal jig whose framework is constructed from welded box section steel. A plurality of pick-up devices is mounted on the framework for carrying the aircraft components during the assembly process, and these too are conventionally produced from welded steel parts.

EP 1 230 124 discloses such an assembly tool.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method of producing a machining fixture for fixedly holding a workpiece during machining of that workpiece, the method comprising: providing an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined; determining a datum, the datum being dependent upon the relative positions of the receiving elements; determining positions and orientations of one or more reference surfaces with respect to the datum; measuring the surface of the initial machining fixture with respect to the datum; and, thereafter, controlling machining apparatus with respect to the datum to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture.

The machining fixture may be an airframe component machining fixture for fixedly holding a workpiece during a process of producing an airframe component from that workpiece.

The method may further comprise providing a first digital model of an object to be produced from the workpiece using the machining fixture. The step of determining positions and orientations of one or more reference surfaces may comprise, using the first digital model, identifying one or more features of the object, and, for each identified feature, determining a position and orientation of a reference surface relative to the datum.

The object to be produced may be an airframe component. Each object feature may be a landing for receiving a further airframe component during an airframe assembly process.

The method may further comprise, using the first digital model and the determined positions and orientations of the one or more reference surfaces, determining a second digital model, the second digital model being a model of the machining fixture specifying the positions and orientations of the reference surfaces. The step of controlling machining apparatus may be performed using the second digital model.

The step of determining the datum may comprise measuring the relative positions of the receiving elements on the initial machining fixture, and, using the measurements of the relative positions of the receiving elements, calculating the datum.

The step of determining the datum may comprise: providing the workpiece to be machined, the workpiece comprising a plurality of attachment features for attaching to the receiving elements of the machining fixture, the relative positions of the attachment features being dependent upon the relative positions of the receiving elements; measuring the relative positions of the attachment features on the workpiece; and, using the measurements of the relative positions of the attachment features, calculating the datum.

In a further aspect, the present invention provides a machining process comprising: producing a machining fixture using a method according to any of the preceding aspects; providing a workpiece to be machined, the workpiece comprising a plurality of attachment features; attaching the workpiece to the machining fixture by attaching the receiving elements of the machining fixture to the attachment features of the workpiece; and, thereafter, performing, using machining apparatus, a machining operation to machine the workpiece so as to produce a predetermined object.

The object may be an airframe component selected from the group of airframe components consisting of a frame and a longeron.

Each reference surface of the machining fixture may be associated with an object feature of the object that is to be produced. Machining the workpiece may comprise, for each reference surface, locating at least part of the machining apparatus against that reference surface, and controlling the machining apparatus to move away from that reference surface with respect to the datum to machine the workpiece so as to form the object feature associated with that reference surface.

Each object feature may be a landing for receiving a further airframe component during an airframe assembly process.

In a further aspect, the present invention provides apparatus for producing a machining fixture for fixedly holding a workpiece during machining of that workpiece, the method comprising: means for providing an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined; one or more processors configured to determine a datum, the datum being dependent upon the relative positions of the receiving elements, and determine positions and orientations of one or more reference surfaces with respect to the datum; measuring apparatus configured to measure the surface of the initial machining fixture with respect to the datum; and machining apparatus configured to be controlled with respect to the datum so as to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture.

In a further aspect, the present invention provides a machining fixture produced using a method according to any of the preceding aspects.

The machining fixture may comprise a rigid base portion, and the receiving elements may extend from an upper surface of the base portion.

The reference surfaces may be formed from sacrificial blocks attached to the base portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration (not to scale) of an example aircraft;

FIG. 2 is a schematic illustration (not to scale) of a portion of an airframe of an aft fuselage of the aircraft;

FIG. 3 is a schematic illustration (not to scale) showing a side view of a first frame;

FIG. 4 is a process flow chart showing certain steps of a process of producing the first frame;

FIG. 5 is a schematic illustration (not to scale) of a perspective view of a frame machining fixture;

FIG. 6 is a schematic illustration (not to scale) showing a top-down view of a workpiece coupled to the frame machining fixture;

FIG. 7 is a process flow chart showing certain steps of an embodiment of a process of producing the airframe;

FIG. 8 is a schematic illustration (not to scale) showing a perspective view of an embodiment of an assembly jig;

FIG. 9 is a schematic illustration (not to scale) showing packer material applied to an outer surface of the components of the airframe;

FIG. 10 is a schematic illustration (not to scale) showing an example way in which the packer material may be applied to a surface of an airframe component;

FIG. 11 is a schematic illustration (not to scale) showing a further example way in which the packer material may be applied to a surface of an airframe component; and

FIG. 12 is a process flow chart showing certain steps of an embodiment of a process of producing the jig.

DETAILED DESCRIPTION

It will be appreciated that relative terms such as horizontal and vertical, top and bottom, above and below, front and back, upper and lower, and so on, are used herein merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented rather than truly horizontal and vertical, top and bottom, and so on.

FIG. 1 is a schematic illustration (not to scale) of an example aircraft 2 comprising an aft fuselage 4, and an aircraft fore section 5. The aircraft fore section 5 includes a central fuselage to which the aft fuselage 4 is attached.

The aft fuselage 4 comprises an airframe and an aircraft skin fastened to the airframe. In this embodiment, the aft fuselage 4 comprises a port boom and a starboard boom that are mechanically attached together along a centreline. Each boom of the aft fuselage 4 comprises a plurality of structural components (for example, frames, keels, longerons, and skins) that are mechanically attached together.

FIG. 2 is a schematic illustration (not to scale) of a portion of the airframe 6 of the aft fuselage 4. In this embodiment, the portion of the airframe 6 is at least a part of a boom of the aft fuselage 4 (i.e. either a port or starboard boom that is to be attached to an opposite boom).

In this embodiment, the airframe 6 comprises three laterally spaced-apart frames or “formers”, namely a first frame 8, a second frame 10, and a third frame 12; three longerons, namely a first longeron 14, a second longeron 16, and a third longeron 18; and a beam 20. For reasons of clarity, the longerons 14, 16, 18 and the beam 20 are shown as hashed in FIG. 2.

In this embodiment, the frames 8, 10, 12 are made of aluminium or titanium. The frames 8, 10, 12 define the shape of the aircraft fuselage and, in use, provide stability to the aircraft 2 by preventing or opposing deflection of the longerons 14, 16, 18. When the aircraft 2 is fully assembled, the frames 8, 10, 12 are arranged substantially perpendicularly to the longitudinal axis of the aircraft 2

In this embodiment, the longerons 14, 16, 18 are made of aluminium or titanium. The longerons 14, 16, 18 are elongate members to which the skin of the aircraft is fastened. When the aircraft 2 is fully assembled, the longerons 14, 16, 18 run substantially parallel to the longitudinal axis of the aircraft 2. In this embodiment, the longerons 14, 16, 18 are fastened to the frames 8, 10, 12 by a plurality of bolts. In this embodiment, the first longeron 14 has a first end 14 a attached to the first frame 8 and a second free end 14 b proximate to the third frame 12. Similarly, the second longeron 16 has a first end 16 a attached to the first frame 8 and a second free end 16 b proximate to the third frame 12. Similarly, the third longeron 18 has a first end 18 a attached to the first frame 8 and a second free end 18 b proximate to the third frame 12.

In this embodiment, during assembly of the aircraft 2, the second free ends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are attached to the fore fuselage 5 of the aircraft 2. In particular, the second free ends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are attached to a frame of the fore fuselage 5. This frame of the fore fuselage 5 to which the second free ends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are to be attached is hereinafter referred to as the “fore fuselage frame”.

In this embodiment, the beam 20 is made of aluminium or titanium. The beam 20 is an elongate member. The beam 20 is attached at its first end (i.e. a proximal end) to the first frame 8 by a plurality of bolts, and extends away from the frames 8, 10, 12 to its second end (i.e. a distal free end) opposite to its first end. When the aircraft 2 is fully assembled, the beam 20 runs substantially parallel to the longitudinal axis of the aircraft 2.

In this embodiment, in addition to being attached together by the longerons 14, 16, 18, the frames 8, 10, 12 are attached together by further structural elements often called “keels” which are located between adjacent frames 8, 10, 12. For ease of illustration and clarity, these keels are not shown in FIG. 2.

In this embodiment, to produce the aft fuselage 4, a composite aircraft skin is fastened to the airframe 6. The outer shape of the assembled aft fuselage 4 (i.e. the outer shape of the aft fuselage 4 produced by fastening the composite skin to the airframe 6) is hereinafter referred to as the Outer Mould Line (OML) of the aft fuselage 4. In this embodiment, the OML of the aft fuselage 4 is to be within a pre-specified tolerance. The OML of the aft fuselage 4 having the required tolerance is facilitated by the Inner Mould Line (IML) of the aft fuselage 4 being within a pre-specified tolerance. The IML of the aft fuselage 4 is the surface at which the airframe 6 and the aircraft skin abut, i.e. an outer surface of the airframe 6 and inner surface of the aircraft skin.

An embodiment of a process of producing the airframe 6 is described in more detail later below with reference to FIG. 7.

FIG. 3 is a schematic illustration (not to scale) showing a side view of the first frame 8.

The first frame 8 comprises a plurality of longeron landings 30 a-c to which, during assembly of the airframe 4, the longerons 14, 16, 18 are fastened. In particular, the first frame 8 comprises a first longeron landing 30 a shaped to receive a portion of the first longeron 14, a second longeron landing 30 b shaped to receive a portion of the second longeron 16, and a third longeron landing 30 c shaped to receive a portion of the third longeron 18. The longeron landings 30 a-c are attachment features to which other components, in particular the longerons 14-18, attach.

The first frame 8 further comprises a plurality of fixture attachment features 32. In this embodiment, there are four fixture attachment features 32. As described in more detail later below, the fixture attachment features 32 are for attaching the first frame 8 to a fixture such as a machining fixture and/or an assembly fixture. In this embodiment, the fixture attachment features 32 are holes through the structure of the first frame 8 through which locator pins or other elongate members may be positioned.

In this embodiment, similarly to the first frame 8, the second frame 10 also comprises three longeron landings, each longeron landing being configured to receive a portion of a respective longeron 14, 16, 18. Also, the second frame 10 comprises a plurality of fixture attachment features.

In this embodiment, similarly to the first and second frames 8, 10, the third frame 12 also comprises three longeron landings, each longeron landing being configured to receive a portion of a respective longeron 14, 16, 18. Also, the third frame 12 comprises a plurality of fixture attachment features.

FIG. 4 is a process flow chart showing certain steps of a process of producing the first frame 8. A similar process, mutatis mutandis, may be used to produce the second and third frames 10, 12. A similar process, mutatis mutandis, may be used to produce the beam 20. A similar process, mutatis mutandis, may be used to produce the longerons 14, 16, 18.

At step s2, a human designer generates or creates a digital model, hereinafter referred to as the “first digital model”. The first digital model is of the first frame 8. This may be performed using a computer and an appropriate software package, for example, the Catia (Trademark) V4 software package. The first digital model may defined using a digital model of the airframe 6 or aft fuselage 4.

At step s4, the human designer generates or creates a further digital model, hereinafter referred to as the “second digital model”. The second digital model is of a frame machining fixture. The frame machining fixture is a fixture system that is to be used to secure a workpiece in place whilst that workpiece is machined to form the first frame 8. The frame machining fixture will be described in more detail later below with reference to FIG. 5.

At step s6, a forging system produces a first forging. The first forging is an aluminium blank from which the first frame 8 is to be produced. A digital model of the first forging may be used to produce the first forging. This digital model may be defined using a digital model of the airframe 6 or aft fuselage 4.

At step s8, using the first digital model, a 5-axis computer numerical control (CNC) milling machine machines the first forging to produce a workpiece from which the first frame 8 is to be produced.

In this embodiment, the workpiece is essentially the same shape as the first frame 8 except that the workpiece comprises additional material where the longeron landings 30 a-c are to be located. The workpiece comprises the fixture attachment features 32. The workpiece will be described in more detail later below with reference to FIG. 5.

At step s10, the forging system produces a second forging. The second forging is a steel blank from which the frame machining fixture is to be produced. In other embodiments, the blank from which the frame machining fixture is to be produced is a structure created by bolting or welding a plurality of substructures together. A digital model of the second forging may be used to produce the second forging. This digital model may be defined using a digital model of the airframe 6 or aft fuselage 4.

At step s12, using the second digital model, the 5-axis CNC milling machine machines the second forging so as to produce the frame machining fixture. In other embodiments, the frame machining fixture is created by bolting or welding a plurality of substructures together. The first digital model may also be used in the production of the frame machining fixture.

FIG. 5 is a schematic illustration (not to scale) of a perspective view of the frame machining fixture 40.

In this embodiment, the frame machining fixture 40 comprises a substantially rigid base portion 41, a plurality of locator pins 42, and a plurality of precision ground blocks 44 a-c.

In this embodiment, there are four locator pins 42. The locator pins 42 are located on an upper surface of the base portion 41 and extend away from the upper surface of the base portion 41 in a direction that is substantially perpendicular to that surface. In this embodiment, each of the locator pins 42 is configured to couple to a respective fixture attachment feature 32 of the workpiece. The relative positions of the locator pins 42 correspond to those of the fixture attachment features 32 such that the workpiece may be placed onto the upper surface of the base portion 41 in such a way that each locator pin 42 couples to a respective fixture attachment feature 32, thereby securing the workpiece against the frame machining fixture 40. The workpiece and the frame machining fixture 40 are complementary. The locator pins 42 are configured to securely hold the workpiece to prevent or oppose movement or deflection of the workpiece while the workpiece is being machined.

In this embodiment, the first and second digital models are created concurrently. This tends to facilitate in the production of the complementary workpiece and the frame machining fixture 40.

In this embodiment, there are three blocks 44 a-c, namely a first block 44 a, a second block 44 b, and a third block 44 c. The blocks 44 a-c are located on an upper surface of the base portion 41 and extend away from the upper surface of the base portion 41 in a direction that is substantially perpendicular to the upper surface of the base portion 41. In this embodiment, the locations of the blocks 44 a-c are such that, when the workpiece is coupled to the frame machining fixture 40 by locating the locator pins 42 in the fixture attachment features 32, each block 44 a-c is proximate to a respective surface of the workpiece that is to be machined so as to form a longeron landing 30 a-c. Furthermore, each block 44 a-c includes a surface that is substantially parallel with a respective surface of the workpiece that is to be machined so as to form a longeron landing 30 a-c. In particular, the first block 44 a is proximate to and substantially parallel with the surface of the workpiece that is to be machined so as to form the first longeron landing 30 a. Similarly, the second block 44 b is proximate to and substantially parallel with the surface of the workpiece that is to be machined so as to form the second longeron landing 30 b. Similarly, the third block 44 c is proximate to and substantially parallel with the surface of the workpiece that is to be machined so as to form the third longeron landing 30 c.

The positions of the blocks 44 a-c in the second digital model may be determined using the positions of the longeron landings 30 a-c in the first digital model.

In some embodiments, a coordinate measuring machine (CMM) is used to inspect the locator pins 42 and/or the blocks 44 a-c. The frame machining fixture 40 may be further processed, i.e. adjusted, to ensure that the frame machining fixture 40 is as specified by the second digital model, for example, the locator pins 42 and/or the blocks 44 a-c may be further machined dependent upon the CMM measurements. This processes of measuring and adjusting the machining fixture may be performed iteratively.

At step s14, a coordinate measuring machine (CMM) measures the relative locations of the fixture attachment features 32 of the workpiece produced at step s8.

At step s16, using the CMM measurements of the fixture attachment features 32 on the workpiece, a computer determines a datum, hereinafter referred to as the “frame datum”. The frame datum is a reference system, with respect to the attachment features, from which measurements may be made. The frame datum may be computed using any appropriate software package, for example, the Valisys (Trademark) software package.

At step s18, the CMM measures the surface of the workpiece with respect to the frame datum. Thus, the locations of points on the surface of the workpiece with respect to the fixture attachment features 32 are determined.

At step s20, the workpiece is fixedly secured to the frame machining fixture 40 by placing the workpiece onto the upper surface of the base portion 41 such that each locator pin 42 is coupled to (i.e. positioned through) a respective fixture attachment feature 32. The locator pins 42 may be threaded and the workpiece may be secured to the threaded locator pins 42 by screwing nuts onto the threaded pins.

FIG. 6 is a schematic illustration (not to scale) showing a top-down view of the workpiece 50 coupled to the frame machining fixture 40.

The three portions of the workpiece 50 that are to be machined so as to produce the longeron landings 30 a-c are hereinafter referred to as the “excess portions” and are indicated in FIG. 6 by hashed regions and the reference numerals 52 a-c. As shown in FIG. 6, the blocks 44 a-c provide surfaces that are proximate to and substantially parallel with the surfaces of the workpiece 50 that are to be machined to form the longeron landings 30 a-c.

At step s22, while the workpiece 50 is attached to the frame machining fixture 40, the 5-axis CNC milling machine machines the workpiece so as to remove the excess portions 52 a-c, thereby forming the longeron landings 30 a-c and producing the first frame.

In this embodiment, the first excess portion 52 a of the workpiece 50 is removed/machined so as to form the first longeron landing 30 a. Also, the second excess portion 52 b of the workpiece 50 is removed/machined so as to form the second longeron landing 30 b. Also, the third excess portion 52 c of the workpiece 50 is removed/machined so as to form the third longeron landing 30 c.

In this embodiment, the removal of the first excess portion 52 a is performed as follows.

Firstly, the 5-axis CNC milling machine probes the surface of the first block 44 a, for example, by moving so as to contact with the surface of the first block 44 a that is parallel with the surface of the workpiece 50 to be machined. In this way, the CNC milling machine determines the location of its cutting tool in space with respect to the frame datum.

In this embodiment, the positional relationship between the blocks 44 a-c and the locator pins 42 is known from the second digital model. Also, when the workpiece 50 is coupled to the frame machining fixture 40, the locator pins 42 are substantially collocated with the fixture attachment features 32 of the workpiece 50. Thus, the positions of the blocks 40 a-c with respect to the frame datum are known. Thus, when CNC machine probes (i.e. contacts with) the first block 44 a, the position of the cutting tool with respect to the frame datum is known/determined with relatively high accuracy.

Secondly, using the known position of the cutting tool, the measurements of the surface of the workpiece taken at step s18, and the first digital model (which specifies the shape of the first longeron landing 30 a), the CNC machine machines away the first excess portion 52 a of the workpiece 50 so as to form the first longeron landing 30 a. In this embodiment, this machining is performed by moving the cutting tool with respect to the frame datum such that the first longeron landing 30 a is formed with a high degree of accuracy relative to the frame datum, i.e. the fixture attachment features 32. Preferably, the cutting tool is moved from the surface of the first block 44 a that is parallel with the surface of the workpiece 50 to be machined, in only a single direction, i.e. along only a single axis. This limited movement of the cutting tool from the block advantageously tends to reduce errors.

The removal of the second and third excess portions 52 b, 52 c to form the second and third longeron landings 30 b, 30 c respectively is performed using an analogous method to that described above for the forming of the first longeron landing 30 a. Thus, in this embodiment, the longeron landings 30 a-c are formed with high accuracy with respect to the frame datum (i.e. the fixture attachment features 32).

Thus, a process of producing the first frame 8 is provided.

In some embodiments, measurements of the positions of the blocks 44 a-c relative to the locator pins 42 may be used to determine the positions of the blocks 44 a-c in the frame datum.

In some embodiments, measurements of the surface of the frame machining fixture 40 (e.g. taken using a CMM) are used to create the “frame datum”. For example, the frame datum may be created using measurements of the locator pins 42 on the frame machining fixture 40.

An embodiment of a process of producing the airframe 6 described in more detail above with reference to FIG. 2 will now be described.

FIG. 7 is a process flow chart showing certain steps of an embodiment of a process of producing the airframe 6.

At step s32, an assembly jig is produced. A process of producing the jig is described in more detail later below with reference to FIG. 12.

FIG. 8 is a schematic illustration (not to scale) showing a perspective view of an embodiment of the jig 60 produced at step s32.

In this embodiment, the jig 60 comprises a steel jig frame 62 comprising a plurality of steel beams that are attached together (e.g. by welding) to form a rectangular parallelepiped.

The jig 60 further comprises a steel reference frame 64 attached (e.g. by welding) to one end of the jig frame 62. In this embodiment, the reference frame 64 is, in effect, a copy of the fore fuselage frame to which, during assembly of the aircraft 2, the aft fuselage 4 is to be attached. In other words, the reference frame 64 is substantially the same as the frame of the fore fuselage 5 to which the second free ends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are to be attached.

The jig 60 further comprises four sets of pickup devices, hereinafter referred to as “jig pickups”. Preferably, the jig pickups are of universal construction. In this embodiment, each jig pickup comprises a mounting element for mounting the device on the jig frame 62, a receiving element for carrying an airframe component, and a plurality of elongate members having predetermined lengths connected together by means of clamping elements so as to allow six degrees of freedom of movement of the receiving element around three orthogonal axes. Examples of appropriate jig pickups include, but are not limited to, those described in EP1230124 and EP1600379, each of which is incorporated in its entirety herein by reference. The pick-up devices may be formed from aluminium.

Each of the first set of jig pickups is indicated in FIG. 8 by the reference numeral 66. Each of the second set of jig pickups is indicated in FIG. 8 by the reference numeral 68. Each of the third set of jig pickups is indicated in FIG. 8 by the reference numeral 70. Each of the fourth set of jig pickups is indicated in FIG. 8 by the reference numeral 72.

As described in more detail later below with reference to FIG. 12, the first jig pickups 66 are configured to securely hold the first frame 8 in a predetermined position relative to the reference frame 64.

In this embodiment, each of the first jig pickups 66 is attached to the jig frame 62, e.g. by bolts or welding. Attachment using bolts advantageously tends to permit adjustment of the first jig pickups 66 so that their positions on the jig frame 62 are in accordance with a digital model that specifies those positions, and also allows for the removal of the first jig pickups 66 from the jig frame 62. Each of the first jig pickups 66 comprises an elongate arm to which is attached a receiving element 74, hereinafter referred to as a “first receiving element”. Each of the first receiving elements 74 is configured to couple to a respective fixture attachment feature 32 of the first frame 8.

For ease of illustration, only two first jig pickups 66 are shown in FIG. 8. However, in reality, the number of first jig pickups 66 is such that the number of first receiving elements 74 is equal to the number of fixture attachment features 32 of the first frame 8.

As described in more detail later below with reference to FIG. 12, the second jig pickups 68 are configured to securely hold the second frame 10 in a predetermined position relative to the reference frame 64.

In this embodiment, each of the second jig pickups 68 is attached to the jig frame 62, e.g. by bolts or welding. Each of the second jig pickups 68 comprises an elongate arm to which is attached a receiving element 76, hereinafter referred to as a “second receiving element”. Each of the second receiving elements 76 is configured to couple to a respective attachment feature of the second frame 10.

For ease of illustration, only two second jig pickups 68 are shown in FIG. 8. However, in reality, the number of second jig pickups 68 is such that the number of second receiving elements 76 is equal to the number of attachment features of the second frame 10.

As described in more detail later below with reference to FIG. 12, the third jig pickups 70 are configured to securely hold the third frame 12 in a predetermined position relative to the reference frame 64.

In this embodiment, each of the third jig pickups 70 is attached to the jig frame 62, e.g. by bolts or welding. Each of the third jig pickups 70 comprises an elongate arm to which is attached a receiving element 78, hereinafter referred to as a “third receiving element”. Each of the third receiving elements 78 is configured to couple to a respective attachment feature of the third frame 12.

For ease of illustration, only two third jig pickups 70 are shown in FIG. 8. However, in reality, the number of third jig pickups 70 is such that the number of third receiving elements 78 is equal to the number of attachment features of the third frame 12.

As described in more detail later below with reference to FIG. 12, the fourth jig pickups 72 are configured to securely hold the second ends of the beams 20, 22 in a predetermined position relative to the reference frame 64.

In this embodiment, each of the fourth jig pickups 72 is attached to the jig frame 62, e.g. by bolts or welding. Each of the fourth jig pickups 72 comprises an elongate arm to which is attached a receiving element 80, hereinafter referred to as a “fourth receiving element”. Each of the fourth receiving elements 80 is configured to couple to a respective attachment feature of a beam 20, 22.

For ease of illustration, only two fourth jig pickups 72 are shown in FIG. 8. However, in reality, the number of fourth jig pickups 72 is such that the number of fourth receiving elements 80 is equal to the number of attachment features of the beams 20, 22.

At step s34, each of the frames 8, 10, 12 are produced as described in more detail earlier above with reference to FIG. 4.

At step s36, the frames 8, 10, 12 are attached to the jig 60.

In particular, in this embodiment the first frame 8 is attached to the first jig pickups 66 such that each of the first receiving elements 74 is coupled to a respective fixture attachment feature 32 of the first frame 8. Thus, the first frame 8 is fixedly attached to the jig 60 at a predetermined position relative to the reference frame 64. In this embodiment, as described in more detail later below with reference to FIG. 12, the position on the jig 60 of the first frame 8 with respect to the reference frame 64 is the same as the desired position on the assembled aircraft 2 of the first frame 8 with respect to the fore fuselage frame.

Also, in this embodiment the second frame 10 is attached to the second jig pickups 68 such that each of the second receiving elements 76 is coupled to a respective attachment feature of the second frame 10. Thus, the second frame 10 is fixedly attached to the jig 60 at a predetermined position relative to the reference frame 64. In this embodiment, as described in more detail later below with reference to FIG. 12, the position on the jig 60 of the second frame 10 with respect to the reference frame 64 is the same as the desired position on the assembled aircraft 2 of the second frame 10 with respect to the fore fuselage frame.

Also, in this embodiment the third frame 12 is attached to the third jig pickups 70 such that each of the third receiving elements 78 is coupled to a respective attachment feature of the third frame 12. Thus, the third frame 12 is fixedly attached to the jig 60 at a predetermined position relative to the reference frame 64. In this embodiment, as described in more detail later below with reference to FIG. 12, the position on the jig 60 of the third frame 12 with respect to the reference frame 64 is the same as the desired position on the assembled aircraft 2 of the third frame 12 with respect to the fore fuselage frame.

At step s38, the beam 20 is attached to the jig 60 and the first frame 8.

In this embodiment, the beam 20 comprises one or more attachment features, i.e. through holes, proximate to its second end. In this embodiment, the beam 20 is attached to the fourth jig pickups 72 such that each of the fourth receiving elements 80 is coupled to a respective attachment feature of the beam 20. Also, the first end of the beam 20 is attached, e.g. by bolts, to the first frame 8. Thus, the beam 20 is fixedly attached to the jig 60 and the first frame 8. In this embodiment, the beam 20 is attached to the jig 60 such that the second end of the beam 20 is at a predetermined position relative to the reference frame 64. In this embodiment, as described in more detail later below with reference to FIG. 12, the position on the jig 60 of the second end of the beam 20 with respect to the reference frame 64 is the same as the desired position on the assembled aircraft 2 of the second end of the beam 20 with respect to the fore fuselage frame.

At step s40, the keels are attached between adjacent frames 8, 10, 12 e.g. by bolting. In this embodiment, the keels not directly attached to the jig 60 e.g. by pickups, and instead are attached to the frames 8, 10, 12 only.

At step s42, the longerons 14, 16, 18 are attached to the jig 60 and the first frame 8.

In particular, in this embodiment, the first longeron 14 is attached to the first longeron landing 30 a of the first frame 8, a first longeron landing of the second frame 10, and a first longeron landing of the third frame 12. Also, the second free end 14 b of the first longeron 14 may be attached to a feature of the reference frame 64 that corresponds to a feature of the fore fuselage frame to which, during assembly of the aircraft 2, the second free end 14 b of the first longeron 14 is to be attached. In some embodiments, the first free end 14 a of the first longeron 14 may be attached to the first frame 8 and/or the jig 60 e.g. to a jig pickup.

Also, in this embodiment, the second longeron 16 is attached to the second longeron landing 30 b of the first frame 8, a second longeron landing of the second frame 10, and a second longeron landing of the third frame 12. Also, the second free end 16 b of the second longeron 16 may be attached to a feature of the reference frame 64 that corresponds to a feature of the fore fuselage frame to which, during assembly of the aircraft 2, the second free end 16 b of the second longeron 16 is to be attached. In some embodiments, the first free end 16 a of the second longeron 16 may be attached to the first frame 8 and/or the jig 60 e.g. to a jig pickup.

Also, in this embodiment, the third longeron 18 is attached to the third longeron landing 30 c of the first frame 8, a third longeron landing of the second frame 10, and a third longeron landing of the third frame 12. Also, the second free end 18 b of the third longeron 18 may be attached to a feature of the reference frame 64 that corresponds to a feature of the fore fuselage frame to which, during assembly of the aircraft 2, the second free end 18 b of the third longeron 18 is to be attached. In some embodiments, the first free end 18 a of the third longeron 18 may be attached to the first frame 8 and/or the jig 60 e.g. to a jig pickup.

As the longeron landings of the frames 8, 10, 12 have been machined with high accuracy with respect to the fixture attachment features 32 of the frames 8, 10, 12 (i.e. to the frame datums), and the frames 8, 10, 12 have been located on jig 60 at predetermined positions with respect to the reference frame 64 using the fixture attachment features 32 of the frames 8, 10, 12, when the longerons 14, 16, 18 are attached to the longeron landings, the longerons 14, 16, 18 tend to be accurately located at predetermined positions with respect to the reference frame 64. In this embodiment, the positions on the jig 60 of the longerons 14, 16, 18 with respect to the reference frame 64 is the same as the desired position on the assembled aircraft 2 of the third frame 12 with respect to the fore fuselage frame.

A step s44, sacrificial plies or layers of material, which are collectively referred to hereinafter as “packer material” are applied, e.g. using an adhesive, to the surfaces of the airframe components (i.e. the surfaces of the frames 8, 10, 12, the longerons 14, 16, 18, and the beam 20) to which the aircraft skin is to be attached.

In this embodiment, the packer material is applied to the airframe components after the airframe 6 has been assembled in the jig 60. However, in other embodiments, the packer material is applied to the airframe components prior to the airframe 6 being assembled in the jig 60.

Steps s46 to s50 of the process of FIG. 7 will be described in more detail later below after a description of the packer material and its application to the airframe components.

FIG. 9 is a schematic illustration showing packer material 90 applied to a surface of the first frame 8 and the first longeron 14.

The desired IML for the airframe 6 is shown in FIG. 9 as a dotted line and is indicated by the reference numeral 92. The IML 92 may be specified in the digital model of the airframe 6 or aft fuselage 4.

In this embodiment, the packer material 90 is made of a composite material such as a fibre-reinforced polymer, i.e. a polymer matrix reinforced with fibres (e.g. carbon or glass fibres). The packer material 90 is a different material to that/those from which the airframe components are made

In this embodiment, the packer material 90 is of sufficient thickness to provide that desired IML 92 is at or beneath the outer surface of the packer material 90. The thickness for the packer material may be determined by performing a tolerance analysis using a detail stage inspection of the components and determined assembly tolerances. In some embodiments, the thickness of the packer material is 1.75 mm to 2.29 mm, e.g. 2.03 mm.

In some embodiments, packer material 90 is not added to some or all of the surfaces of the airframe components. For example, in some embodiments, if the outer surface of an airframe component is located at the desired IML 92, packer material 90 is not added to that surface.

The application of the packer material 90 to the components of the airframe 6 may be performed in any appropriate way. In some embodiments, the packer material 90 comprises flat sheets which are pressed against and attached to the components of the airframe 6. Alternatively, the packer material 90 may be shaped to fit against, i.e. be complementary to, the components of the airframe 6. For example, the packer material 90 may be moulded into an appropriate shape by applying the packer material 90 to a mould tool and curing the packer material 90 in an autoclave.

FIG. 10 is a schematic illustration showing an example way in which the packer material 90 may be applied to an outer surface of the first frame 8.

In this embodiment, an adhesive 94 is used to adhere the packer material 90 to the first frame 8. An over press tool 96 is used to hold the packer material 90 in place while the adhesive 94 cures or solidifies.

In this embodiment, the over press tool 96 comprises a rigid body portion 98 (also referred to as a “strong back”) and a deformable portion 100. The body portion 98 is substantially non deformable. The body portion 98 comprises a plurality of clamp receiving elements 102 for receiving respective clamp devices 104 such as G-clamps.

In use, the over press tool 96 is positioned onto a portion of the airframe 6 such that packer material 90 is between the over press tool 96 and the first frame 8, and such that the deformable portion 100 of the over press tool 96 is in contact with the packer material 90. The over press tool 96 is then attached to and pressed against the packer material 90 and first frame 8 using a plurality of clamp devices 104 which engage with the clamp receiving elements 102 of the body portion 98 and the first frame 8. The over press tool 96 remains in place until the adhesive 94 has cured and the packer material 90 is securely attached to the first frame 8, at which point the clamp devices 104 and the over press tool 96 are removed.

In this embodiment, the over press tool 96 is shaped to be complementary to a part of the first frame to which the packer material is to be applied. The first digital model (i.e. the digital model of the first frame 8) may be used to specify a shape for the over press tool 96 and/or specify a digital model for producing the over press tool 96. The body portion 98 may then be produced using an appropriate process, such as an Additive Layer Manufacturing (ALM) process, from any appropriate material, such as plastic.

The deformable portion 100 advantageously tends to provide that the force exerted by the over press tool 96 onto the packer material 90 is substantially evenly distributed. Furthermore, the deformable portion 100 tends to facilitate the use of the over press tool 96 in cases where there are assembly positioning errors etc.

In this example, the over press tool 96 is held against the packer material 90 and first frame 8 using a plurality of clamp devices 104. However, in other examples, the over press tool 96 may be held against the packer material 90 and an airframe component in a different way.

FIG. 11 is a schematic illustration showing a further example way in which the packer material 90 may be applied to an outer surface of the first frame 8.

In this embodiment, the over press tool 96 further comprises a rigid arm 106 connecting the body portion 98 to an actuation device 108. The actuation device 108 may, for example be a pneumatic or hydraulic actuation device 108 configured to actuate the over press tool 96. The over press tool 96 further comprises a vacuum line 110 connecting the surface of the deformable portion 100 to a vacuum pump 112.

In operation, the first frame 8 is securely held by a support structure 114 (e.g. the jig 60). The packer material 90 is held onto the deformable portion 100 of the over press tool 96 by establishing a vacuum in the vacuum line 110. The over press tool 96 is then actuated by the actuation device 108 in such a way that the packer material 90 coupled to the over press tool 96 is brought into contact with the first frame 8 (i.e. the over press tool 96 is moved in the direction of the arrows in FIG. 11) and firmly pressed against the first frame 8 until the adhesive 94 cures. The vacuum holding the packer material 90 to the over press tool 96 may then be released and the over press tool 96 may be moved away from the first frame 8 leaving the packer material 90 adhered thereto.

Having the packer material 90 be retained against the over press tool 96 and subsequently bringing the packer material 90 coupled to the over press tool 96 into contact with the first frame 8 (as opposed to applying the packer material 90 to the first frame and subsequently bringing the over press tool into contact with the packer material 90 on the first frame 8) advantageously tends to reduce unwanted movement of the packer material 90 with respect to the first frame 8. For example, a likelihood of the packer material 90 that is applied to the first frame 8 moving with respect to the first frame 8 when the over press tool 96 is brought into contact with the packer material 90 on the first frame 8 tends to be reduced or eliminated. Thus, accuracy with which packer material 90 is applied to an object tends to be increased.

Furthermore, having the packer material 90 be retained against the over press tool 96 and subsequently bringing the packer material 90 coupled to the over press tool 96 into contact with the first frame 8 tends to reduce the likelihood of the packer material 90 adhering to the first frame 8 before a pressing force is applied. This tends to improve adherence of the packer material to the object.

Returning now to the process of FIG. 7, at step s46, optionally, a laser tracker or CMM is used to measure the outer surface of the packer material 90 that has been applied to the components of the airframe 6. In this embodiment, the outer surface of the packer material 90 is measured with respect to a so-called “jig datum” which will be described in more detail later below with reference to FIG. 12. In some embodiments, this step may be omitted or may be performed at a later stage.

At step s48, a CNC cutting device is used to machine the packer material 90 such that the outer surface of the packer material 90 is located at the IML 92 specified in the digital model of the airframe 6 or aft fuselage 4 with respect to the fore reference frame 64. Optionally, this may be performed using the laser tracker measurements taken at step s46. In this embodiment, the CNC cutting device is controlled with respect to the “jig datum” *which is described in more detail later below.

Thus, the packer material 90 is machined to allow for variations in component thicknesses and substructure assembly positioning errors. This tends to provide that, when the aircraft skin is attached to the airframe 6, the OML of the aft fuselage 4 is within the pre-specified tolerance with respect to the fore reference frame 4 (i.e. fore fuselage in the assembled aircraft 2). Furthermore, the machined packer material 90 provides a consistent landing surface to which the aircraft skin may be fixed. The landing surface provided by the machined packer material 90 is accurately positioned with respect to the jig datum.

Use of the sacrificial packer material 90 advantageously tends to reduce or eliminate a need to machine the surfaces of the airframe components (i.e. the surfaces of the frames 8, 10, 12, the longerons 14, 16, 18, and the beam 20) to which the aircraft skin is to be attached. Thus, a need for performing post-machining analysis/testing of the airframe components may be avoided or reduced.

Use of the packer material 90 advantageously tends to reduce or eliminate a need for shims to be applied to fill gaps between the airframe 6 and the external aircraft skin when the aircraft skin is attached to the airframe 6.

Steps s46 and s48 may be performed iteratively.

Thus, the airframe 6 is assembled using the jig 60.

At step s50, the assembled airframe 6 may be removed from the jig 60.

Thus, an embodiment of a process of producing the airframe 6 is provided.

Returning now to the process of producing the jig 60 performed at step s32, FIG. 12 is a process flow chart showing certain steps of an embodiment of a process of producing the jig 60.

At step s50, a plurality of steel beams are attached together, e.g. by welding, to form a rectangular parallelepiped, thereby producing the jig frame 62. An example jig frame and method of construction thereof includes, but is not limited to, those described in EP1230124 and EP1600379, each of which is incorporated in its entirety herein by reference. The pick-up devices may be formed from aluminium.

At step s52, the reference frame 64 is attached, e.g. by welding, to one end of the jig frame 62. In this embodiment, the reference frame 64 is a substantially exact replica of at least part of the fore fuselage frame to which the airframe 6 of the aft fuselage 4 is to be attached.

At step s54, a CMM measures a surface of the reference frame 64 on the jig 60. In particular, in this embodiment, the CMM measures features of the reference frame 64 that correspond to those features of the fore fuselage frame to which the airframe 6 is to be attached.

At step s56, using the CMM measurements of the reference frame 64, a computer determines a datum, herein referred to as the “jig datum”. The jig datum is a reference system, with respect to the reference frame 64, from which measurements may be made. The jig datum may be computed using any appropriate software package.

At step s58, a human designer generates or creates a further digital model, hereinafter referred to as the “third digital model”. The third digital model is of the airframe and the fore fuselage frame. The third digital model specifies the positional relationships between the components of the airframe 6 and the fore fuselage frame, when the airframe 6 is attached to the fore fuselage frame.

The third digital model may be created using a computer and an appropriate software package, for example, the Catia (Trademark) V4 software package.

At step s60, using the third digital model, the human designer or a computer determines positions on the jig frame 60 for each of the jig pickups 66, 68, 70, 72.

In this embodiment, the determined locations on the jig 60 of the first jig pickups 66 are such that, were the first frame 8 to be held by the first jig pickups 66, the position of the first frame 8 relative to the reference frame 64 would be substantially the same as a desired position for the first frame 8 relative to the fore fuselage frame onboard the assembled aircraft 2.

Similarly, the determined locations on the jig 60 of the second jig pickups 68 are such that, were the second frame 10 to be held by the second jig pickups 68, the position of the second frame 10 relative to the reference frame 64 would be substantially the same as a desired position for the second frame 10 relative to the fore fuselage frame onboard the assembled aircraft 2.

Similarly, the determined locations on the jig 60 of the third jig pickups 70 are such that, were the third frame 12 to be held by the third jig pickups 70, the position of the third frame 12 relative to the reference frame 64 would be substantially the same as a desired position for the third frame 12 relative to the fore fuselage frame onboard the assembled aircraft 2.

Similarly, the determined locations on the jig 60 of the fourth jig pickups 72 are such that, were the second ends of the beams 20, 22 held by the fourth jig pickups 72, the positions of the second ends of the beams 20, 22 relative to the reference frame 64 would be substantially the same as a desired position for the second ends of the beams 20, 22 relative to the fore fuselage frame onboard the assembled aircraft 2.

At step s62, each of the jig pickups 66-72 are attached to the jig frame 62, e.g. by bolts or by welding, in the position determined at step s60 for that jig pickup.

Steps s64 and s66 are optional steps for verifying and, if necessary, adjusting the position and/or orientation of the jig pickups on the jig 60. In some embodiments, these steps are omitted or a different verification and/or different adjustment process is performed. Steps s64 and s66 may be performed iteratively.

At step s64, a laser tracker measures the position of each of the jig receiving elements 74-80 with respect to the jig datum, i.e. with respect to the fore reference frame 64.

At step s66, using the laser tracker measurements, the positions of the jig receiving elements 74-80 with respect to the jig datum may be modified to ensure that each of the jig receiving elements 74-80 has the position determined at step s60 for that jig receiving element 74-80. In other words, the positions of the jig receiving elements 74-80 may be modified to ensure that, when the frames 8, 10, 12 are held by the jig receiving elements 74-80, the positions of the frames 8, 10, 12 relative to the reference frame 64 are substantially the same as a desired position for the frames 8, 10, 12 relative to the fore fuselage frame onboard the assembled aircraft 2.

Modifying a position of a jig receiving element 74-80 may be performed by, for example, introducing a shim between a jig pickup and the jig frame 62, or by machining part a jig pickup and/or the jig frame 62.

In some embodiments, the positions of one or more of the jig pickups 66, 68, 70, 72 measured by the laser tracker may be used to update or recalculate the jig datum. For example, in some embodiments, a new jig datum is calculated using measured positions of the reference frame and one or more of the jig receiving elements 74-80. This new jig datum is a reference system, with respect to the reference frame 64 and one or more of the jig receiving elements 74-80, from which measurements may be made.

Thus, an embodiment of a process of producing the jig 60 is provided.

An advantage provided by the above described methods and apparatus is that the airframe advantageously is produced within very tight tolerance bounds that tend not to be possible using conventional production techniques. The airframe is produced with high accuracy relative to the reference frame which is representative of a forebody frame to which the airframe is to be attached. In particular, the airframe is produced such that the IML is within very with tight tolerances with respect to the reference frame. Thus, the assembly of the aircraft, and in particular the attaching together of the aft fuselage and the fore fuselage, tends to be facilitated.

Advantageously, the blocks of the frame machining fixture are precision ground with respect to the locator pins of the frame machining fixture (i.e. the frame datum). During production of the frame, each block is used as a “zero point” from which a cutting tool is moved to machine a respective frame longeron landing close to that block. The proximity of the block to the associated longeron landing advantageously tends means that, in order to form a longeron landing, the cutting tool does not have to be moved large distances from a “zero point”. Thus, errors tends to be reduced and the accuracy (with respect to the frame datum) with which the longeron landings are formed tends to be increased.

Advantageously, the above described jig tends to use fewer pickups for holding the airframe components compared to conventional assembly jigs. Thus, the weight and cost of the jig tends to be reduced compared to conventional assembly jigs. Furthermore, as airframe components are held by fewer pickups, damage/stresses resulting from a component being held by pickups tends to be reduced.

Advantageously, the longeron landings of the frames are machined with high accuracy with respect to a local frame datum that is defined by frame features that are used to locate that frame in the assembly jig. Thus, because the jig pickups that couple to the frame attachment features are accurately located on the jig in a desired position with respect to the reference frame, when the frames are attached to the jig, the longeron landings are accurately located on the jig in a desired position with respect to the reference frame. Thus, when the longerons are attached to the longeron landings of the frames, the longerons tend to be are accurately located on the jig in a desired position with respect to the reference frame. Accordingly, gaps or spaces between longerons and frames advantageously tend to be minimised. The use shims to fill such gaps or spaces tends to be reduced or eliminated.

The above described methods and apparatus tends to provide that the frames of the airframe may be produced independently from one another. In other words, there tends to be no requirement to machine or process the frames as a set. This independent production of the frames tends to reduce production time of the airframe compared to conventional processes.

A further advantage provided by the above described methods and apparatus is that non-destructive testing and other processes may be performed on individual airframe components separately. This tends to be difficult if, in contrast to the above described method, multiple airframe components are machined as a set in an assembly jig.

Apparatus, including the any of the abovementioned computers or processes for performing any of the above described data processing method steps may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and/or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media.

It should be noted that certain of the process steps depicted in the flowcharts of FIGS. 4, 7, and 9 and described above may be omitted or such process steps may be performed in differing order to that presented above and shown in FIGS. 4, 7, and 9. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally.

In the above embodiments, an airframe of an aircraft aft fuselage is produced. However, in other embodiments, a different type of structure is produced, for example a structure that, in use, is to be attached to a different structure may be produced. For example, an airframe of a different part of the aircraft, e.g. the fore fuselage, may be produced. In such embodiments, the jig may comprise a different type of reference frame representing a different type of entity to which the structure being assembled is to be attached.

In the above embodiments, the aircraft comprises three frames, three longerons, two beams, and a plurality of keels. However, in other embodiments, the airframe comprises a different number of frames, longerons, beams, and/or keels. In some embodiments, one or more of the listed types of airframe components may be omitted. In some embodiments, the airframe comprises a different type of airframe component instead of or in addition to those listed above.

In the above embodiments, a frame comprises four attachment features which are used to determine the frame datum. The frame attachment features are holes through the structure of the frame. Also, the jig comprises jig pickups configured to attach to the frame attachment features. Also, the frame machining fixture comprises locator pins configured to couple to the frame attachment features. However, in other embodiments, one or more of the frames or other airframe components comprises a different number of attachment features. In some embodiments, one or more of the attachment features may be a different type of attachment feature other than a through hole. Also, the jig may comprise a different type of pickup that is configured to attach to the different type of attachment feature. For example, in some embodiments, an attachment feature may be a block-like structure and a jig pickup may comprise a clamp for clamping to the block-like structure. Also, the frame machining fixture may comprise a different type of device that is configured to attach to the different type of attachment feature.

In the above embodiments, longeron landings are machined with respect to a local frame datum. However, in other embodiments one or more different types of frame features are formed with respect to the frame datum instead of or in addition to one or more of the longeron landings.

In the above embodiments, the airframe components are made of aluminium. However, in other embodiments, one or more of the airframe components is made of a different type of material.

In the above embodiments, the jig frame is a steel beam frame in the shape of a rectangular parallelepiped. However, in other embodiments, the jig frame is made of a different material and/or is a different shape.

In the above embodiments, the frame machining fixture comprises three blocks which are used as reference points or surfaces from which a CNC cutting tool is moved. The blocks are located on an upper surface of the frame machining fixture. In other embodiments, the frame machining fixture comprises a different number of blocks or other devices that provide the above described functionality. In some embodiments, one or more of the blocks is located on a different part of the frame machining fixture, e.g. on a side of the base portion. 

What is claimed is: 1: A method of producing a machining fixture for fixedly holding a workpiece during machining of that workpiece, the method comprising: providing an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined; determining a datum, the datum being dependent upon relative positions of the receiving elements; determining positions and orientations of one or more reference surfaces of the machining fixture with respect to the datum; measuring a surface of the initial machining fixture with respect to the datum; and thereafter, controlling machining apparatus with respect to the datum to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture. 2: The method according to claim 1, wherein the machining fixture is an airframe component machining fixture configured for fixedly holding the workpiece during a process of producing an airframe component from that workpiece. 3: The method according to claim 1, wherein: the method comprises providing a first digital model of an object to be produced from the workpiece using the machining fixture; and the step of determining positions and orientations of one or more reference surfaces comprises: using the first digital model, identifying one or more features of the object; and for each identified feature, determining a position and orientation of a reference surface relative to the datum. 4: The method according to claim 3, wherein: the machining fixture is an airframe component machining fixture configured for fixedly holding the workpiece during a process of producing an airframe component from that workpiece; and each object feature is a landing for receiving a further airframe component during an airframe assembly process. 5: The method according to claim 1, wherein the method further comprises, providing a first digital model of an object to be produced from the workpiece using the machining fixture; and using the first digital model and the determined positions and orientations of the one or more reference surfaces, determining a second digital model, the second digital model being a model of the machining fixture specifying the positions and orientations of the reference surfaces; and the step of controlling machining apparatus is performed using the second digital model. 6: The method according to claim 1, wherein the step of determining the datum comprises: measuring the relative positions of the receiving elements on the initial machining fixture; and using the measurements of the relative positions of the receiving elements, calculating the datum. 7: The method according to claim 1, wherein the step of determining the datum comprises: providing the workpiece to be machined, the workpiece comprising a plurality of attachment features for attaching to the receiving elements of the machining fixture, relative positions of the attachment features being dependent upon the relative positions of the receiving elements; measuring the relative positions of the attachment features on the workpiece; and using the measurements of the relative positions of the attachment features, calculating the datum. 8: A machining process comprising: producing a machining fixture using the method according to claim 1; providing a workpiece to be machined, the workpiece comprising a plurality of attachment features; attaching the workpiece to the machining fixture by attaching the receiving elements of the machining fixture to the attachment features of the workpiece; and thereafter, performing, using machining apparatus, a machining operation to machine the workpiece so as to produce a predetermined object. 9: The process according to claim 8, wherein the object is an airframe component selected from the group of airframe components consisting of a frame and a longeron. 10: The process according to claim 8, wherein each reference surface of the machining fixture is associated with an object feature of the object that is to be produced; and machining the workpiece comprises, for each reference surface: locating at least part of the machining apparatus against that reference surface; and controlling the machining apparatus to move away from that reference surface with respect to the datum to machine the workpiece so as to form the object feature associated with that reference surface. 11: The process according to claim 10, wherein: the object is an airframe component selected from the group of airframe components consisting of a frame and a longeron; and each object feature is a landing for receiving a further airframe component during an airframe assembly process. 12: An apparatus for producing a machining fixture for fixedly holding a workpiece during machining of that workpiece, the machining fixture being formed from an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined, the apparatus comprising: one or more processors configured to: determine a datum, the datum being dependent upon relative positions of the receiving elements; and determine positions and orientations of one or more reference surfaces with respect to the datum; measuring apparatus configured to measure a surface of the initial machining fixture with respect to the datum; and machining apparatus configured to be controlled with respect to the datum so as to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture. 13: A machining fixture produced using the method of claim
 1. 14: The machining fixture according to claim 13, wherein the machining fixture comprises a rigid base portion, and the receiving elements extend from an upper surface of the base portion. 15: The machining fixture according to claim 14, wherein the reference surfaces are formed from sacrificial blocks attached to the base portion. 